Japanese nuclear regulators raised the severity of the latest leak of radiation-contaminated water at a damaged plant to the level of "serious incident" on Wednesday.

Plant operator Tokyo Electric Power Co said last week about 300 tons of highly contaminated water had leaked from a storage tank at the Fukushima Daiichi plant.

The Nuclear Regulation Authority raised the severity of the leak on the International Nuclear Event Scale from level 1 - an "anomaly" - to level 3.

The leak was the worst since the start of the nuclear disaster in 2011, the operator said.

Level 7 - "major accident" - was applied when three of the reactors suffered meltdowns.

Fukushima toxic leak is much worse than authorities state – nuclear expert

The current water leaks at Fukushima are “much worse than we were led to believe,” said an independent consultant on nuclear issues.

Water is leaking out all over the site and there are no accurate figures for radiation levels, stated Mycle Schneider, lead author for the World Nuclear Industry status reports. He has previously advised the French and German governments and a variety of organizations.

"The quantities of water they are dealing with are absolutely gigantic. Nobody can measure that," Mr. Schneider said.

According to him, the toxic water is leaking out not just from the tanks but also from the basements and the cracks all over the place.

There are also worries about the spent nuclear fuel rods that are being cooled and stored in water pools on site. Mycle Schneider says these contain far more radioactive caesium than was emitted during the explosion at Chernobyl.

"There is absolutely no guarantee that there isn’t a crack in the walls of the spent fuel pools. If salt water gets in, the steel bars would be corroded. It would basically explode the walls, and you cannot see that; you can’t get close enough to the pools," he said.

Japanese nuclear regulators said Wednesday they planned to lift the severity of the latest leak of radioactive water at a damaged plant in Fukushima to the level of "serious incident."

Tokyo Electric Power Co said a day earlier that about 300 tons of highly-contaminated water had leaked from a storage tank at the Fukushima Daiichi Nuclear Power Station.

Some of the water might have flown into the Pacific Ocean, the company said Wednesday.

The Nuclear Regulation Authority said it planned to raise the severity on the International Nuclear Event Scale (INES) from level 1 - an "anomaly" - to level 3.

The leak was "the worst" since the start of the nuclear disaster in 2011, the operator said.

Level 7 - "major accident" - was applied in 2011.

Hundreds of storage tanks in the plant were designed to contain large amounts of contaminated water from three reactors that suffered meltdowns after the complex was hit by the March 2011 earthquake and tsunami.

"The IAEA (the International Atomic Energy Agency) views this matter seriously and remains ready to provide assistance on request," Gill Tudor, a spokeswoman, said in a statement.

"Japanese authorities continue to provide the Agency with information on the situation at the plant, and Agency experts are following the issue closely," she said.

Japan needs help from outside to stabilize reactors at Fukushima NPP

The operator of the crippled Fukushima nuclear plant has said they need help from outside Japan to stabilize and safely decommission damaged reactors at the facility.

This follows the news that regulators are poised to declare a fresh toxic water leak at Fukushima a level 3 "serious incident," the gravest warning since the massive 2011 earthquake and tsunami that sent three reactors into meltdown

Japan nuclear agency concerned about more Fukushima leaks

Japan’s nuclear watchdog said on Wednesday it is concerned that more storage tanks at the wrecked Fukushima nuclear plant will spring leaks, following the discovery that highly contaminated water is leaking from one of the hastily built containers.

The deepening crisis at the Fukushima plant will be upgraded from a level 1 "anomaly" to a level three "serious incident" on an international scale for radiological releases, a spokesman for Japan’s Nuclear Regulation Authority said earlier on Wednesday.